THE BRACHIAL PLEXUS 357 



which join the spinal accessory within the sternomastoid and 

 trapezius, and also between these two muscles. 



The INTERNAL are: Communicating, from the loop between 

 the first and second, to the vagus, hypoglossal, and sympa- 

 thetic, and a branch from the fourth to the fifth. 



Muscular, to the lateral and anterior recti muscles (from 

 the first and second). 



Communicantes hypoglossi, generally two, one from the second 

 and one from the third, pass under or over the internal jugular 

 to join the descending branch from the hypoglossal nerve. 



Phrenic, from the third, fourth, and fifth, descends on the 

 scalenus anticus, then between the subclavian artery and 

 vein, and crosses the internal mammary artery. It then crosses 

 in front of the root of the lung and runs between the peri- 

 cardium and mediastinal pleura to the diaphragm; it com- 

 municates with the sympathetic, descendens noni, and the 

 nerve to the subclavius. The right is deeper than the left. It 

 runs external to the innominate vein and superior vena cava. 

 The left crosses the front of the aortic arch and the left vagus. 

 Both phrenics supply the diaphragm, pleura, and pericardium. 

 Filaments from the right, with the phrenic branches of the 

 solar plexus, form a ganglion, which sends branches to the 

 suprarenal capsules and inferior vena cava and to the hepatic 

 plexus; on the left side there is no ganglion. 



The Brachial Plexus 



The brachial plexus is formed by the anterior divisions of 

 the lower four cervical and first dorsal, as follows: The fifth 

 and sixth form an upper; the seventh, a middle; and the eighth 

 cervical with first dorsal, a lower trunk. Each of these trunks 

 then separates into an anterior and a posterior branch. 



The anterior branches of the upper and middle trunks form 

 the outer cord of the plexus; the anterior branch of the lower, 

 the inner cord; of the posterior cord it is variously stated that 

 the posterior branches of all three trunks form it, or that the 

 posterior branches of the upper and middle trunks form it, 

 while the posterior branch of the lower trunk joins the musculo- 

 spiral nerve. It is altogether a matter of dissection. 



The plexus is at first between the anterior and middle scaleni, 

 then above and external to the subclavian artery. It passes 



